cmTarget: Short-circuit language computation if context independent.

Computing the language involves computing the source files, which
is an expensive operation.  It requires calling
cmMakefile::GetOrCreateSource many times, which involves creating
and matching on many cmSourceFileLocation objects.

Source files of a target may depend on the head-target and the
config as of commit e6971df6 (cmTarget: Make the source files depend
on the config., 2014-02-13).  The results are cached for each context
as of commit c5b26f3b (cmTarget: Cache the cmSourceFiles in
GetSourceFiles., 2014-04-05).

Each target in the build graph causes language computation of all
of its dependents with itself as the head-target.  This means that
for 'core' libraries on which everything depends, the source files
are computed once for every transitive target-level-dependee and
the result is not cached because the head-target is different. This
was observed in the VTK buildsystem.

Short circuit the computation for targets which have a source-list
that is independent of the head-target.  If the source-list has
already been computed and the generator expression evaluation
reports that it was context-independent, return the only source-list
already cached for the target.  Reset the short-circuit logic when
sources are added and when the link libraries are re-computed.

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